About 20% of energy consumption in Germany is for the generation of light. Conventional incandescent lamps are inefficient, and the most efficient fluorescent lamps contain up to 10 mg of mercury. Solid-state lighting devices, such as, for example, light-emitting diodes (LEDs), are a highly promising alternative, since they have better efficiency in the conversion of electrical energy into light (energy efficiency), a longer lifetime and higher mechanical stability than conventional light sources. LEDs can be used in multifarious applications, including displays, motor vehicle and sign illumination and domestic and street lighting. Depending on the inorganic semiconductor compound used for its production, an LED can emit monochromatic light in various regions of the spectrum. However, “white” light, which is necessary for much of the lighting industry, cannot be generated using a conventional LED. Current solutions for the generation of white light include either the use of three or more LEDs having different colours (for example red, green and blue or “RGB”) or the use of a colour conversion layer comprising a conventional phosphor material (for example YAG:Ce) for the generation of white light from ultraviolet (UV) or blue emission of an LED. Thus, blue light is converted into light having a longer wavelength, and the combination of blue and yellow light is perceived as white light by the human eye. However, white light of this type is virtually never ideal and has in many cases undesired or unpleasant properties, which may require improvement or correction. The simpler construction of conversion LEDs is directed to the mass market of lighting devices. At present, these LED lamps are still significantly more expensive than conventional incandescent lamps and most fluorescent lamps, and the commercially available white LEDs emit a bluish, cold-white light having poor colour reproduction properties. This quality of white light which is perceived as poor originates from the yellow conversion phosphor material YAG:Ce owing to the lack of emission in the green and red parts of the spectrum.
For displays, it is important to have three or more primary colours having a narrow spectral full width at half maximum (FWHM), which are obtained with LEDs (typical FWHM<30 nm). This enables a large gamut to be covered. “Gamut” is normally defined as the range of colour types which is obtainable by mixing three colours. However, the solution of using three or more LEDs of different colours is too expensive and complex for many applications. It is therefore desirable to have available a light source which enables large gamut coverage using a single LED, which can be achieved by conversion materials which emit in a narrow band. A process for the provision of LEDs for a light source having a broad spectrum utilises phosphors which convert the short-wave LED light into light having a longer wavelength. For example, a phosphor which emits light over a broad range of green wavelengths can be excited using blue light from an LED which generates a narrow blue spectrum. The green light generated by the phosphor is then used as a component of the white light source. By combining a plurality of phosphors, a white light source having a broad spectrum can in principle be created, provided that the efficiencies of the phosphors during the light conversion are sufficiently high. This would result in improved colour reproduction properties. Further details can be found in “Status and prospects for phosphor-based white LED Packaging”, Z. Liu et al., Xiaobing Front. Optoelectron. China 2009, 2(2): 119-140.
Unfortunately, however, a light designer does not have access to any desired set of phosphors from which he is able to select. There is only a limited number of conventional rare-earth element-containing phosphors which can be employed in LEDs and which have adequate efficiencies in light conversion. The emission spectrum of these phosphors cannot readily be modified. In addition, the spectra are less than ideal inasmuch as the light emitted as a function of the wavelength is not constant. Even combination of a plurality of phosphors therefore does not produce an optimum white light source. In addition, red phosphors currently used emit light deep into the long-wave red spectral region, which additionally reduces the brightness of such LEDs and thus their efficiency.
U.S. Pat. Nos. 7,102,152, 7,495,383 and 7,318,651 disclose devices and processes for the emission of light in which both semiconductor nanoparticles in the form of quantum dots (QDs) and also non-quantum fluorescent materials are utilised in order to convert at least some of the light originally emitted by a light source of the device into light having a longer wavelength. QDs have a high quantum yield and a narrow emission spectrum with a central emission wavelength which can be adjusted via the size. A combination of both QDs and phosphors enables the light quality to be improved. QD additions enable improvements to be achieved, but have the disadvantage of high inherent absorption, i.e. they absorb light which is emitted when they are excited themselves. This reduces the overall energy efficiency of the light conversion. In addition, QDs, like commercially available red emitters, likewise reabsorb the green phosphor emission, which additionally results in a decrease in the energy efficiency and in addition in a shift in the emission spectrum, making targeted colour planning more difficult. In addition, separation may occur during production of LEDs when QD materials and phosphors are used, meaning that a homogeneous distribution of the light-converting materials is no longer ensured. Reduced energy efficiency and inadequate control of the desired colour reproduction are the consequence.
In some applications, clusters with tightly packed QDs are desired. Tightly packed QD clusters of this type exhibit a phenomenon which is known under the name fluorescence resonance energy transfer (FRET), see, for example, Joseph R. Lakowicz, “Principles of Fluorescence Spectroscopy”, 2nd Edition, Kluwer Academic/Plenum Publishers, New York, 1999, pp. 367-443. FRET occurs between a donor QD, which emits with a shorter (for example bluer) wavelength, and an acceptor QD, which is arranged in the direct vicinity and emits with longer wavelength. A dipole-dipole interaction occurs between the dipole moment of the donor emission transition and the dipole moment of the acceptor absorption transition. The efficiency of the FRET process depends on the spectral overlap between the absorption of the donor and the emission of the acceptor. The FRET separation between quantum dots is typically 10 nm or less. The efficiency of the FRET effect is very highly dependent on the separation. FRET results in a colour change (red shift) and in efficiency losses during light conversion. For this reason, efforts were made in earlier work to avoid cluster formation of QDs in light-converting materials.
Semiconductor nanoparticles are a class of nanomaterials whose physical properties can be adjusted over a broad range by adjustment of the particle size, composition and shape. In particular, the fluorescence emission is one of the properties of this class which is dependent on the particle size. The adjustability of the fluorescence emission is based on the quantum limitation effect, according to which a reduction in the particle size results in “particle in a box” behaviour, which results in a blue shift of the band gap energy and thus in light emission. Thus, for example, the emission of CdSe nanoparticles can be adjusted from 660 nm for particles having a diameter of ˜6.5 nm to 500 nm for particles having a diameter of ˜2 nm. Similar behaviour can be achieved for other semiconductor nanoparticles, which results in it being possible to cover a broad spectral range from the ultraviolet (UV) region (on use of, for example, ZnSe or CdS) via the visible (VIS) region (on use of, for example, CdSe or InP) to the near infrared (NIR) region (on use of, for example, InAs). A change in the shape of the nanoparticles has already been demonstrated for a number of semiconductor systems, where, in particular, the rod shape is of importance. Nanorods have properties which differ from the properties of spherical nanoparticles. For example, they exhibit emission which is polarised along the longitudinal axis of the rod, whereas spherical nanoparticles have unpolarised emission. Anisotropic (elongate) nanoparticles, such as nanorods (also sometimes referred to as “rods” below), are thus suitable for polarised emission (see WO 2010/095140 A3). X. Peng et al. in “Shape control of CdSe nanocrystals” in Nature, 2000, 404, 59-61, describe CdSe nanorods which are embedded in a polymer and which are based on a colloid-based semiconductor core (without shell). Virtually complete polarisation emanates from individual nanorods here. T. Mokari and U. Banin in “Synthesis and properties of CdSe/ZnS rod/shell nanocrystals” in Chemistry of Materials 2003, 15 (20), 3955-3960, describe an improvement in the emission of nanorods if a shell is applied to the rod structure. D. V. Talapin et al. in “Seeded Growth of Highly Luminescent CdSe/CdS Nanoheterostructures with Rod and Tetrapod Morphologies” in Nano Letters 2007, 7 (10), 2951-2959, describe an improvement in the quantum yield achieved for nanorod particles having a covering (shell). C. Carbone et al. in “Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeled Growth Approach” in Nano Letters 2007, 7 (10), 2942-2950, describe a dipole emission pattern for nanorods having a shell, which means that the emission emanates from the centre of the rod instead of from its tips. In addition, it has been shown that nanorods have advantageous properties with respect to optical amplification, which illustrates their potential for use as laser materials (Banin et al., Adv. Mater., 2002, 14, 317). For individual nanorods, it has likewise been shown that they exhibit unique behaviour in external electrical fields, since the emission can be switched on and off reversibly (Banin et al., Nano Letters, 2005, 5, 1581). A further attractive property of colloidal semiconductor nanoparticles is their chemical accessibility, which allows these materials to be processed in various ways. The semiconductor nanoparticles can be applied in the form of thin layers from solution by spin coating or spray coating or embedded in plastics. Jan Ziegler et al. in “Silica-Coated In P/ZnS Nanocrystals as Converter Material in White LEDs”, Advanced Materials, Vol. 20, No. 21, 13 Oct. 2008, pages 4068-4073, describe the production of white LEDs with the addition of a silicone composite layer which comprises a light-emitting conversion material on a high-performance blue LED chip.
The use of semiconductor nanoparticles in LED applications has been described, inter alia, in US 2015/0014728 A1, which relates to a phosphor/matrix composite powder which can be used in an LED. The phosphor/matrix composite powder comprises a matrix and a plurality of phosphors or quantum dots having a size of 100 nm or less which are dispersed in the matrix, where the size of the composite powder is 20 μm or more and it has a certain surface roughness. Even during its production, the composite powder described requires precise setting of the mixing ratios of the phosphors and quantum dots in order to achieve the desired emission behaviour. By contrast, subsequent adaptation of the mixing ratios is not possible, which results in restricted flexibility in the usability of the composite powder in the production of LEDs. In addition, the efficiency of the energy conversion is highly dependent on the type and amount of the phosphor materials dispersed in the matrix. In particular if the amount of the phosphors and/or quantum dots is large, it becomes difficult to sinter the material. In addition, the porosity increases, which makes efficient irradiation with excitation light more difficult and impairs the mechanical strength of the material. If, however, the amount of the phosphor materials dispersed in the matrix is too low, it becomes difficult to achieve adequate light conversion.
In view of the numerous deficiencies of the known light-conversion materials mentioned above, including the known combinations of QDs with conventional phosphors (conversion phosphors), there is a need for semiconductor nanoparticle materials and compositions which comprise such materials with conventional phosphors which do not have such deficiencies. In particular, there is a need for combinations of semiconductor nanoparticles and conversion phosphors having low to negligible reabsorption and having low inherent absorption, which results in high efficiency in the conversion of light and in improved controllability of the gamut.
It would therefore be desirable to have available light-converting materials based on semiconductor nanoparticles which are distinguished by low reabsorption and inherent absorption and thus increase the energy efficiency of an LED. Furthermore, it would be desirable to have available light-converting materials based on semiconductor nanoparticles which are distinguished by improved miscibility with conventional phosphors, so that the disadvantages described above (for example reduced energy efficiency and inadequate control of colour reproduction) which arise owing to separation effects during LED production are avoided.